1. Technical Field of Invention
The present invention relates to the recovery of ethane and heavier components from hydrocarbon gas streams. More particularly, the present invention relates to recovery of ethane and heavier components from hydrocarbon streams utilizing multiple reflux streams.
2. Description of Prior Art
Valuable hydrocarbon components, such as ethane, ethylene, propane, propylene and heavier hydrocarbon components, are present in a variety of gas streams. Some of the gas streams are natural gas streams, refinery off gas streams, coal seam gas streams, and the like. In addition these components may also be present in other sources of hydrocarbons such as coal, tar sands, and crude oil to name a few. The amount of valuable hydrocarbons varies with the feed source. The present invention is concerned with the recovery of valuable hydrocarbon from a gas stream containing more than 50% methane and lighter components [i.e., nitrogen, carbon monoxide (CO), hydrogen, etc.], ethane, and carbon dioxide (CO2). Propane, propylene and heavier hydrocarbon components generally make up a small amount of the overall feed. Due to the cost of natural gas, there is a need for processes that are capable of achieving high recovery rates of ethane, ethylene, and heavier components, while lowering operating and capital costs associated with such processes. Additionally, these processes need to be easy to operate and be efficient in order to maximize the revenue generated form the sale of NGL.
Several processes are available to recover hydrocarbon components from natural gas. These processes include refrigeration processes, lean oil processes, refrigerated lean oil processes, and cryogenic processes. Of late, cryogenic processes have largely been preferred over other processes due to better reliability, efficiency, and ease of operation. Depending of the hydrocarbon components to be recovered, i.e. ethane and heavier components or propane and heavier components, the cryogenic processes are different. Typically, ethane recovery processes employ a single tower with a reflux stream to increase recovery and make the process efficient such as illustrated in U.S. Pat. No. 4,519,824 issued to Huebel (hereinafter referred to as “the '824 patent”); U.S. Pat. No. 4,278,457 issued to Campbell et al.; and U.S. Pat. No. 4,157,904 issued to Campbell et al. Depending on the source of reflux, the maximum recovery possible from the scheme may be limited. For example, if the reflux stream is taken from the hydrocarbon gas feed stream or from the cold separator vapor stream, or first vapor stream, as in the '824 patent, the maximum recovery possible by the scheme is limited because the reflux stream contains ethane. If the reflux stream is taken from lean residue gas stream, then 99% ethane recovery is possible due to the lean composition of the reflux stream. However, this scheme is not very efficient due to the need to compress residue gas for reflux purposes.
A need exists for a process that is capable of achieving high ethane recovery, while maintaining its efficiency. It would be advantageous if the process could be simplified so as to minimize capital costs associated with additional equipment.